feat: allow capture to override timestamp with datetime or datetimeoffset

[darkopia]

my previous attempt changed the wrong place but should likely remain.

I am trying to add timestamp override support to .Net from this support ticket.

Other SDK's support sending timestamps as a property instead of using current datetime.

[greptile-apps]

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[haacked]

Mind adding a unit test to cover this new behavior?

[haacked]

Rather than try to extract timestamp from properties, we should add one or more extension method overrides of Capture to src/PostHog/Capture/CaptureExtensions.cs that takes in a timestamp and passes it along to the properties.

[haacked]

Nice work!